Process for obtaining triggering signals by the comparison of currents at the ends of a power transmission path

ABSTRACT

The invention relates to a differential protective system for electric power transmission paths where measurement quantities obtained from the currents at either end of the section to be monitored are digitized and subjected to digital sine filtering. Sign signals are obtained from auxiliary measured quantities generated from the filtered current measurement values and taken to both ends of the section to be monitored for evaluation with the generation of an exciting signal. To make the system largely unaffected by frequency variations in the currents, further filtered current measurement values are obtained by cosine filtering; from those values and from the individual filtered current measurement values, a measurement quantity is generated by absolute value generation and low-pass filtering. This measurement quantity causes a further exciting quantity to be established using an absolute value comparison quantity. If both exciting signals are present, the sign signals are evaluated.

This application is a 371 of PCT/DE93/00791 filed Aug. 25, 1993.

BACKGROUND OF THE INVENTION

The invention relates to a process for obtaining triggering signals bythe comparison of currents at the ends of a section to be monitored ofan electric power transmission path. More particularly, the presentinvention relates to a process for obtaining triggering signals where atboth ends of the section to be monitored the measurement quantitiesobtained from the currents are phase-selectively digitized, digitalcurrent measurement values are obtained, and from which subsequentlyfiltered current measurement values are obtained by sine filtering. Astored reference current measurement value is subtracted from thepresent filtered current measurement value, forming an auxiliarymeasurement quantity. A sign signal denoting the sign of the currentauxiliary measured quantity is retained for evaluation at one end and isalso transmitted through a data transmission path for evaluation to theother end of the section to be monitored. An exciting quantity providingthe current evaluation is obtained if the auxiliary measured quantity isgreater than a previously determined threshold value.

A known process of this type is described in the Siemens DifferentialCurrent Protection System 7 SD 51 and its basic process steps aredescribed in the journal "Elektrie," 45(1991)7, pp. 272-276. Thiscurrent differential protection system is a fast-acting selectiveshort-circuit protection system for cables or aerial lines which formpower transmission paths. In this current differential protectionsystem, the direct components contained in the monitored current aresuppressed and the harmonics dampened. Also, in this differentialprotection system, a filtered current measurement value, detected twoperiods of the current earlier on the transmission path, or a predefinedcurrent threshold value is used to build the auxiliary measuredquantity. The sign signals, formed at both ends of the section to bemonitored of the electric power transmission path from the auxiliarymeasured quantities, are compared at either end as long as the auxiliarymeasured quantity is greater than a predefined threshold value.Depending on the sign of the sign signals that are compared to eachother, a triggering counter is activated during evaluation; thisactivation occurs by addition or subtraction of constants havingdifferent values depending on whether the comparison results in the needfor quick triggering or for a high degree of stabilization.

An object of the present invention is to configure the process describedabove so that it operates with a high degree of reliability even whenthe current frequency on the section to be monitored of an electricpower transmission path changes relatively quickly.

SUMMARY OF THE INVENTION

This and other objects are achieved by the method of the presentinvention. The present invention provides for subjecting the digitalcurrent values to digital cosine filtering at both ends of the sectionto be monitored, thus additional filtered current values are obtained.An absolute value measurement quantity is generated from the first andadditional filtered current values through absolute value generation andlow-pass filtering. A stored absolute value reference measurementquantity is subtracted from the actual value of the absolute valuemeasurement quantity, thus an absolute value comparison quantity isgenerated. The absolute value comparison quantity is compared to apredefined threshold value and another exciting quantity is produced ifthe absolute value of the absolute value comparison quantity is greaterthan the threshold value. Also, the sign signal with the correspondingsign signal from the other end of the section to be monitored areevaluated if both exciting quantities are present.

An important advantage of the process of the invention is itsconsiderable independence with respect to frequency. This means thateven in the case of relatively rapid frequency changes in the current onthe section to be monitored of an electric power transmission path, thesign signals are only evaluated on the basis of the simultaneouspresence of both exciting quantities if actual currents of the order ofmagnitude of short-circuit currents flow through the section to bemonitored. This is explained by the fact that in the process of theinvention, an absolute value measurement quantity is generated inaddition to the auxiliary measurement quantity used for generating anabsolute value comparison quantity using a stored absolute valuereference measurement quantity; the reference measurement quantity islargely independent of the frequency changes in the currents on thesection to be monitored. The second exciting quantity is obtained bycomparing the absolute value comparison quantity with a predefinedthreshold value. This second exciting quantity is thus also largelyindependent of the frequency changes in the currents on the section tobe monitored. Hence, the process of the present invention ensures thatwhen a second exciting quantity is present in addition to the firstexciting quantity, the sign signal is actually evaluated only due toflowing short-circuit currents.

In the process of the present invention, different absolute valuemeasurement quantities can be used as absolute value referencemeasurement quantities. For sufficiently reliable operation, however,the use of an absolute value measurement quantity detected two periodsof the current earlier is considered to be advantageous if this absolutevalue measurement quantity is smaller than a predefined absolute valuethreshold value; if the absolute value measurement quantity is greaterthan a predefined absolute value threshold value, the predefinedabsolute value threshold value is used as the absolute value referencemeasurement quantity.

In order to further enhance the reliability of the process according tothe present invention, the first exciting quantity is suppressed if thedifference between a present digital current measurement value and aprevious, stored digital current measurement value is smaller than apredefined minimum value. This allows one to safely determine whether ornot the section to be monitored of the electric power transmission pathshould be considered de-energized; by suitably choosing the predefinedminimum value it is ensured that interference will not affect theresult.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an arrangement for implementing the processaccording to the present invention.

FIG. 2 shows the frequency response of the sine filtering performed inFIG. 1.

FIG. 3 shows the frequency response of the cosine filtering performed inFIG. 1.

FIG. 4 shows the frequency response of the absolute value generationperformed in FIG. 1.

FIG. 5 is a diagram further illustrating the mode of operation of theprocess according to the present invention.

DETAILED DESCRIPTION

As shown in FIG. 1, currents J1, J2, and J3 of a section 1 to bemonitored of an electric power transmission path (otherwise notillustrated here) are detected by transducers 2, 3, and 4. Measurementquantities M1, M2, and M3, obtained from currents J1, J2, and J3 aresupplied, via additional matching transducers 5, 6, and 7 to amultiplexer 8, followed by an analog-to-digital converter 9. Thus,digital current values J(t) are obtained at the output of this converter9.

These digital current measurement values J(t) are subjected to sinefiltering in a sine filter 10, so that filtered current measurementvalues i_(s) (t) are obtained at the output of this sine filter 10.These values i_(s) (t) can be described by the following equation (1)

    i.sub.s (t)=i.sub.s (t-1)+J(t)-2J(t-1/2T)+J(t-T)           (1)

In this equation, T provides the period of currents J1 through J3 insection 1 to be monitored. The frequency response of sine filter 10 isshown in FIG. 2.

The present filtered current measurement values, i_(s) (t), are furtherprocessed in a modifying module 11 so that a stored current referencemeasurement value is obtained from the present filtered currentmeasurement value, while an auxiliary measurement quantity i*(t) isgenerated simultaneously.

The auxiliary measuring quantity i*(t) is given by the followingrelationship (2):

    i*(t)=i.sub.s (t)-i.sub.s (t-2T)                           (2)

if relationship (3) exists:

    -I.sub.thresh <i.sub.s (t-2T)<+I.sub.thresh.               (3)

In this equation (3), the quantity I_(thresh) provides a predefinedthreshold value whose value is selected in conformance with the maximumload current of the line to be protected.

If the stored absolute value current measurement reference valuesatisfies the following relationship (4)

    i.sub.s (t-2T)>+I.sub.thresh,                              (4)

then the auxiliary measuring quantity i*(t) is defined by the followingrelationship (5):

    i*(t)=i.sub.s (t)-I.sub.thresh.                            (5)

If the stored absolute value current measurement reference valuesatisfies the following relationship (6):

    -I.sub.thresh >i.sub.s (t-2T).                             (6)

then the auxiliary measuring quantity is defined by the followingequation (7):

    i*(t)=i.sub.s (t)+I.sub.thresh.                            (7)

The auxiliary measuring quantities generated in this manner are checkedin a sign builder 12 for their sign, and sign signals V_(p) (t) andV_(n) (t) are generated; these signals have the value "1" if thefollowing relationships exist (equations (8) and (9)):

    V.sub.p (t)=1 if i*(t)>+I.sub.sign                         (8)

    V.sub.n (t)=1 if i*(t)<-I.sub.sign.                        (9)

I_(sign) represents a threshold for determining the direction of thecurrent and is selected analogously to similar threshold quantities usedin known phase comparison devices which also evaluate a currentdirection using operational signs, but which, instead of the amountI*(t), use the measured current I(t) directly.

Sign signals V_(p) (t) and V_(n) (t) are supplied to a data transmissionsection 13 and an evaluation block 14. As further illustrated in FIG. 1,the auxiliary measurement quantity i*(t) is supplied to a thresholdvalue device 15, where it is determined whether the auxiliarymeasurement quantity i*(t) exceeds a predefined threshold value. Anexciting quantity S_(M) (t) appears at the output of threshold valuedevice 15 if the following conditions defined by equations (10) and (11)below are satisfied:

    S.sub.M (t)=1 if i*(t)>+I.sub.prot.thresh.                 (10)

    S.sub.M (t)=1 if i*(t)<-I.sub.prot.thresh..                (11)

The variable I_(prot).thresh represents an initial threshold for theprotection process of the present invention.

The exciting quantity S_(M) (t) is supplied to another threshold valuedevice 16.

As shown in FIG. 1, the analog-to-digital converter 9 is followed by alow-pass filter 17, where the digital current value J(t) is filtered,and a difference is generated between the present digital currentmeasurement value J(t) and a previous, stored digital currentmeasurement value. An output value J_(d) is thus obtained, which issupplied to a recognition module 18. In this recognition module 18, itis checked whether this additional quantity J_(d) is smaller than apredefined minimum value. If it is not smaller than the minimum value, asignal N(t) is supplied to threshold value device 15, whereupon thefirst exciting quantity S_(M) (t) is forwarded to the second thresholdvalue device 16.

FIG. 1 also shows that the digital current measurement values J(t) arealso supplied to a cosine filter 19 (the frequency response of thecosine filter 19 is illustrated in FIG. 3), in which a current i_(c) (t)is generated according to the relationship below (12):

    I.sub.c (t)=i.sub.c (t-1)+J(t)-2J(t-1/4T)+2J(t-3/4T)-J.sub.n (t-T)(12)

The filtered current measurement values i_(c) (t) are supplied to anabsolute value generator 20, to which the sine-filtered currentmeasurement values i_(s) (t) are also supplied. In this absolute valuegenerator 20 an absolute value i_(B) (t) is generated according to thefollowing equation (13): ##EQU1##

Absolute value generator 20 also performs a low-pass filtering, so thatat the output of absolute value generator 20 an absolute valuemeasurement quantity i_(BF) (t) appears, which can be described by thefollowing equation (14) (the frequency response without taking intoconsideration low-pass filtering is shown by FIG. 4): ##EQU2##

In the following module 21, an absolute value comparison quantity i*_(B)(t) is generated using an absolute value reference measurement quantity,if the relationships of the following equations (15) and (16) aresatisfied:

    i*.sub.B (t)=i.sub.BF (t)-i.sub.BF (t-2T), if i.sub.BF (t-2T)<+I.sub.Bthresh(15)

    i*.sub.B (t)=i.sub.BF (t)-i.sub.Bthresh, if i.sub.BF (t-2T)>+I.sub.thresh.(16)

In both of these equations (15) and (16), I_(Bthresh) denotes apredefined current threshold value, while i_(BF) (t-2T) denotes anabsolute value reference measurement quantity, detected two currentperiods before. Equation (16) shows that the current threshold valueI_(Bthresh) is also used as an absolute value reference quantity.

A subsequent, additional threshold value device 22 checks whether theabsolute value comparison quantity i*_(B) (t) is greater than athreshold value I_(Bthresh), whose value is selected in conformance withthe maximum load current of the line. If this is the case, then theoutput signal S_(B) (t) of this additional threshold value device 22 is"1," which signals that the threshold value has been exceeded. Thesecond threshold value device 16 then receives this output signal as asecond exciting quantity S_(B) (t).

If both exciting quantities S_(M) (t) and S_(B) (t) are present, thenevaluation block 14 is enabled for evaluating the sign signals by beingsupplied with an output signal S(t) from the second threshold valuedevice 16. For this purpose further sign signals G_(p) (t) and G_(n)(t), transmitted by a device having the general design of the deviceillustrated in FIG. 1 to the other end (not illustrated) of line section1 to be monitored, are needed; these sign signals are captured via asecond data transmission section 23.

Evaluation block 14 contains as an essential component an electroniccounter, whose count status Z(t) is changed according to signals S(t),V_(p) (t), V_(n) (t) G_(p) (t), and G_(n) (t) as shown in the last threecolumns of FIG. 5. In particular, this figure shows that, regardless ofthe sign signals V_(p) (t), V_(n) (t), G_(p) (t), and G_(n) (t), thecounter is incremented by one when signal S(t) is "0" and the countstatus was previously smaller than "0." If the count status Z(t) wasgreater than "0" under the same circumstances, then the counter isdecremented by "1." On the other hand, if signal S(t) is "1," then thecounter value is changed as shown in the next-to-last column of FIG. 5.This means that if the sign of V_(p) (t) and G_(p) (t) is positive, thecounter is decremented by "8" for high triggering. If V_(p) (t) is "1"if and G_(p) (t) is "0," then the counter is decremented by "4" only,for low triggering. Otherwise high stabilization occurs throughincrementation by "7" if V_(p) (t)=1 and G_(n) (t)=1. The behavior ofthe counter for other different sign combinations can be easily seenfrom the other lines of FIG. 5. In specific cases, the counter is set sothat for a status Z(t)=-13 triggering occurs, which is signaled by theappearance of a signal at output 24. This signal can actuate a powerswitch not illustrated in the figure via known actuating contacts.

In a preferred embodiment for carrying out the process of the invention,the different modules 8 through 22 of the device of FIG. 1 areimplemented by a data processing system.

What is claimed is:
 1. A method of obtaining a triggering signal bycomparing currents at ends of a section to be monitored of an electricpower transmission path, wherein, at either end of said section to bemonitored, each of a plurality of measurement quantities obtained fromsaid currents is phase-selectively subjected to analog-to-digitalconversion whereby digital current measurement values are obtained, andwhereby first filtered current measurement values are obtained from saiddigital current measurement values via digital sine filtering the methodcomprising:(a) generating an auxiliary measuring quantity by subtractinga stored current reference measurement value from a present value ofsaid first filtered current measurement value; (b) retaining a signsignal that denotes a sign of a present value of said auxiliarymeasuring quantity, said sign signal being evaluated at one end of saidsection to be monitored; (c) transmitting said sign signal via a datatransmission path to the other end of said section to be monitored wheresaid sign signal is evaluated; (d) obtaining a first exciting quantityif an absolute value of said auxiliary measuring quantity is greaterthan a predefined threshold value; (e) subjecting said digital currentmeasurement values to digital cosine filtering such that additionalfiltered current measurement values are obtained; (f) generating anabsolute value measuring quantity from said first and additionalfiltered current measurement values via absolute value generation andlow-pass filtering operations; (g) generating an absolute valuecomparison quantity by subtracting a stored absolute value measuringquantity from a present value of said absolute measuring quantity; (h)comparing said absolute value comparison quantity with a firstpredefined threshold value, such that a second exciting quantity isobtained if an absolute value of the absolute value comparison quantityis greater than said first predefined threshold value; and (i)evaluating said sign signal and corresponding sign signals from saidother end of said section to be monitored when both of said first andsecond exciting quantities are present.
 2. The method of claim 1,wherein in step (g) said stored absolute value measuring quantity is anabsolute measuring quantity that is detected two periods of saidcurrents earlier if said absolute measuring quantity that is detectedtwo periods of said currents earlier is less than a second, predefinedabsolute value threshold value, and said stored absolute value measuringquantity is said second, predefined absolute value threshold value ifsaid absolute value measuring quantity that is detected two periods ofsaid currents earlier is greater than said second, predefined absolutevalue threshold value.
 3. The method of claim 1 wherein said firstexciting quantity is suppressed if a difference between a present valueof said digital current measurement values and a previous, stored valueof said digital current measurement values is smaller than a predefinedminimum value.
 4. The method of claim 2 wherein said first excitingquantity is suppressed if a difference between a present value of saiddigital current measurement values and a previous, stored value of saiddigital current measurement values is smaller than a predefined minimumvalue.